1. Field of the Invention
This invention relates generally to series/parallel LED drive systems, and more particularly to techniques designed to optimize the power efficiency of such systems.
2. Description of the Related Art
LED lighting strategies may employ LEDs driven in series, parallel, or both. LEDs driven in series by definition all share the same current. If all LEDs share the same current, ideally the brightness of the LEDs will be matched. Some applications require a number of LEDs to be driven with matched brightness, and so connecting the LEDs in series accomplishes the task. A problem can arise, however, if a very large number of LEDs must be driven in series. The series-connected LEDs are powered by a line voltage necessary to provide the necessary current; however, finding line regulators able to support the large line voltage needed for a high LED count series string may be difficult or cost prohibitive.
LEDs may also be arranged in parallel-connected ‘strings’, each of which is driven by a current source or (most commonly) a current sink circuit. But brightness matching between the parallel-connected LEDs is limited by the imperfect matching of the drive circuits, which can vary widely depending on the choice of sink implementation. A parallel LED configuration does have the advantage of typically requiring a lower line voltage than does a series configuration, which may be a benefit in some applications. Also, in some applications LEDs are connected in parallel because different currents need to be driven through the LEDs.
Due to the issues noted above, the best approach may be a compromise between the series and parallel solutions: a “series/parallel” solution. Note that a series/parallel solution could in principle be implemented by simply taking the series approach discussed above and creating multiple copies of this solution. However, this cut and paste approach adds cost to the overall solution because of the need for separate line regulators for each string (or “channel”). In some cases a single integrated circuit (IC) with multiple regulator channels may be able to take the place of multiple regulators, but for a number of solutions an appropriate multiple output regulator may not exist or may still be cost prohibitive due to the number of non-regulator external components required.
A cost effective compromise employing a series/parallel solution is shown in FIG. 1. Here, each series LED string 10, 12, 14 has its own independent current sink, but at the same time all series strings share a common line voltage Vline driven from a single line regulator 16. The conventional solution for choosing an appropriate line voltage value recognizes that the LED sink devices (NMOS FETs M1, M2 and M3 in FIG. 1, but other devices might also be used) are easiest to work with when all the devices operate in their active region. An NMOS FET will operate in its active region so long as its drain-source voltage (Vds) is sufficiently large. Thus, for a series/parallel LED solution such as that shown in FIG. 1, insuring that each sink device operates in active mode amounts to choosing a sufficiently large line voltage.
In order to maximize the power efficiency of a series/parallel LED solution such as that shown in FIG. 1, one would ideally like to choose the line voltage—here set by a voltage divider 18—such that all sink NMOS devices operate with just enough drain-source voltage to operate in active mode. Some power must be dissipated in the current sinks in order to achieve the desired LED drive currents, but ultimately any power dissipated in the sinks above the power required by the LEDs represents efficiency loss. To minimize the efficiency loss from current sink dissipation, the drain-source voltage of the sink devices should be designed to be as small as possible. This is, the power Psink dissipated in each current sink, given by Psink=Isink*Vds where Isink and Vds are the current conducted by and the voltage across the sink device, respectively, should be as small as possible.
Ideally, the line voltage is sufficient to guarantee nominal active operation for the NMOS sink devices, but also large enough to account for variations in the components (such as between the drain-source voltages needed for active operation of the sink devices and between the forward voltage drops of the LEDs). One technique used to achieve this utilizes a “minimum” circuit to dynamically account for variation between the forward voltage drops; this approach is illustrated in FIG. 2. The minimum circuit 20 receives the drain voltage of each of the NMOS current sinks and outputs the minimum drain voltage of the group. A line control amplifier 22 receives the minimum voltage and a reference voltage Vref-drain at respective inputs, and provides an output to the feedback input of line regulator 16 such that the LED channel with the minimum drain voltage operates at a desired target voltage equal to Vref-drain. The drain reference voltage is typically chosen to be just large enough to guarantee that the sink device operates in its active region. Because all LED strings share a common line voltage, it can be inferred that the channel with the minimum drain voltage possesses the largest voltage drop across its string of LEDs. Therefore, once the system regulates the minimum drain voltage to the target voltage, it can be inferred that the remaining non-minimum channels have sink devices operating in their active regions because the voltage drop across the LED strings in each of those channels is known to be smaller than the voltage drop of the LED string of the minimum channel.
The minimum circuit solution of FIG. 2 works well in cases where the sink devices are well-matched. However, to ensure reliable operation, Vref-drain needs to be large enough to account for any variation between the sink devices over process and temperature. To improve the power efficiency of the system in FIG. 2, Vref-drain could be lowered below the point where the sink devices operate in active, so that they instead operate in their linear regions (also referred to herein as operating in the “triode region” or simply “in triode”). The first problem that arises with this strategy is that any sink device operating in triode will now have its current strongly dependent upon both its gate-source voltage and its drain-source voltage. Even in active mode, the sink device currents are weakly dependent upon drain-source voltage. A local closed loop around each sink device eliminates or at least reduces this dependence, and can help ensure that the current through the device regulates to the desired value. One example of a popular local closed loop topology is shown in FIG. 3 (which depicts only the LED string 10 portion of FIG. 2), which can be realized in either discrete or IC contexts. Here, sink device M0 is connected in series with a resistance R0 at a junction 30, and a “local current loop amplifier” 32 drives sink device M0 with a voltage Vg0 as needed to make the voltage at junction 30 equal to a reference voltage Vref, such that LED string 10 conducts a desired current given by Vref/R0. One advantage of the topology in FIG. 3 is the use of a resistance (R0) to set the current through the sink device and LED string, as resistors generally match well in IC designs and thus provide for good sink-to-sink current matching.